1. Field of the Invention
The present invention relates to a polymer solid-electrolyte composition having improved ion conductivity, and to an improvement of an electrochemical cell using it.
2. Description of the Background Art
It is known that perfluorocarbon sulfonic acid, polysulfones, perfluorcarbonic acid, styrene-divinylbenzene sulfonic acid cation-exchange resins and styrene-butadiene anion-exchange resins are available as conventional polymer solid electrolytes. Particularly, perfluorocarbon sulfonic acid, a fluorinated resin developed by DuPont and known under the trade name of NAFION.RTM., is noted as a polymer solid electrolyte which is excellent in chemical stability and heat resistance. NAFION.RTM. comprises the base of the copolymer of tetrafluoroethylene and perfluorovinyl ether, on which some sulfonate groups are present as ion-exchange functional groups.
Among them, the granular polymer solid electrolyte is packed in a column and the like, and used for the purification of water and others. A membrane, in which a polymer solid electrolyte is prepared in the form of film has been used as an ion-exchange membrane type of oxygen sensor and an ion-exchange membrane of electrochemical cells such as the salt-electrolysis and water-electrolysis cells, besides the fuel cell. A study has also been carried out to determine whether the membrane can be used as the ion-exchange membrane in an ion-exchange membrane cell for acetaldehyde synthesis. Among these electrochemical cells, particularly the fuel cell, which converts directly chemical energy from hydrogen, reformed natural gas, and the like to electrical energy, generates cleaner energy and has higher efficacy of power generation compared with other generators such as an internal combustion engine, a gas turbine or a diesel engine; and therefore it is noted to use as power source for an electric vehicle, a space craft and the like.
The above-mentioned electrochemical cells are briefly described as follows. The fuel cell converts chemical energy into electrical energy; the ion-exchange membrane type of oxygen sensor and the like measures the amount of a substance into information; and the water electrolysis cell and the like converts electrical energy into chemical energy. In all cases, the donation-acceptance reaction of electrons on an anode or cathode electrode produces an ionic current flow in the polymer solid-electrolyte membrane (in the ion-exchange membrane). This ionic current is produced by the movement of cations or anions from an anode to a cathode, or that of cations or anions from a cathode to an anode. Since this movement of ions is carried out through ion-exchange functional groups such as sulfonic acid, amine or carboxyl groups, the higher the ion-exchange functional group content becomes, the smaller the electrical resistance, i.e., the specific resistance will be less. If water occupies the space between the ion-exchange functional groups, the water molecules are thought to move. When polymer solid electrolytes are dried, their ion conductivity tends to decrease markedly. Therefore, better moisture absorption of a polymer solid electrolyte prevents its ion conductivity from decreasing. This maintenance of ion conductivity makes the energy loss smaller when a current flows in the polymer solid electrolyte, then a high-performance electrochemical cell can be obtained. The membrane where moisture is sufficiently absorbed also prevents the reaction gases from passing through the membrane directly.
Thus, in order to obtain a high-performance electrochemical cell, it is required to improve the ion conductivity by increasing the water content of the polymer solid electrolyte as well as by lowering the specific resistance thereof. Hitherto, improvement methods have been proposed as follows:
1) The method for improving the ion conductivity of the polymer solid-electrolyte membrane by increasing the level of ion-exchange functional groups in polymer solid electrolytes including hydrocarbon ion-exchange resins, fluorinated resins such as NAFION.RTM. (the trade name of perfluorocarbon sulfonic acid developed by DuPont in the U.S.A.), and the like so as to lower the specific resistance of the polymer solid-electrolyte membrane. PA1 2) The method for improving the ion conductivity of the polymer solid-electrolyte membrane by mechanically processing the membrane prepared with the polymer solid electrolytes to make the membrane thinner so as to lower the resistance of the polymer solid electrolyte membrane. PA1 3) The method for improving the ion conductivity of the polymer solid-electrolyte membrane by putting moisture-transportive fibers in the form of twisted yarn to moisten the polymer solid-electrolyte membrane through the fibers, thereby making the moisture absorption of the membrane improve by increasing the water content of the polymer solid-electrolyte membrane. PA1 4) The method for improving the ion conductivity of the polymer solid-electrolyte membrane by moistening the reaction gas at the anode side or cathode side with water vapor or water droplets so as to make the membrane moisten indirectly, as in the case of an electrochemical cell as well as a fuel cell to which the reaction gas is supplied.
If the number of the ion-exchange functional groups in the polymer solid electrolytes is increased, the mechanical characteristic of the polymer is lost in the cases of the hydrocarbon ion-exchange resins; and thus there arises a problem that it is difficult to produce the electrochemical cells because the polymer solid-electrolyte membrane is not easily joined to the anode or cathode. Further, as the fluorinated membranes such as NAFION.RTM. and the like become fluidized, it is difficult technically to introduce a number of ion-exchange functional groups.
On the other hand, if the thickness of the polymer solid-electrolyte membrane is made smaller, the membrane itself has low mechanical strength; and therefore the polymer solid-electrolyte membrane is easily damaged.
Furthermore, the membrane can be moistened by the method in which fibers in the form of twisted yarn, are placed into the membrane, but the increased membrane thickness due to the thickness of the sandwiched fibers leads to a decrease of the ion conductivity.
Furthermore, the method in which the reaction gas is moistened with water vapor or water droplets brings about a decrease in the partial pressure of the reaction gas because the partial presure of the reaction gas is diluted by the water vapor content, and this results in a factor that decreases the electrochemical cells' performance due to dilution of the reaction gas in the catalyst layer. Since it is difficult to vary the amount of water vapor required by the membrane depending on the fluctuating load, insufficient supply with water makes the membrane dry, or on the contrary, excess supply with water makes the catalyst layer too wet. As a result, the cell's performance is degraded.
Consequently, it is necessary to develop polymer solid electrolytes having improved ion conductivity which increase the electrochemical cells' performance since membranes prepared with conventional polymer solid electrolytes have difficulties when used as ion-exchange membranes in operating cells.